The following relates generally to nuclear medicine medical imaging. It finds particular application in conjunction with Positron Emission Tomographic (PET) detectors and localization of positron annihilation events, and will be described with particular reference thereto. However, it will be understood that it also finds application in other usage scenarios and is not necessarily limited to the aforementioned application.
In PET imaging, a subject is injected with a radiopharmaceutical targeting metabolic processes. The radiopharmaceutical accumulates in the targeted tissues and emits positrons as the radiopharmaceutical decays. The positron interacts with an electron in an annihilation event, which generates two gamma photons of 511 keV energy directed 180° opposite to each other. During the period of pharmaceutical decay, the subject is placed in an imaging device or scanner, which includes detectors that detect the generated gamma photons. The imaging device typically includes rings of detectors which encircle a length of the subject.
The detectors detect each gamma photon at a detector location or pixel, with a time and an energy level. Time windows and energy windows are applied to detected photons to determine coincident event pairs of gamma photons, i.e. two photons from the same annihilation event. Coincident event pairs define the lines of response (LORs), which are used to localize the annihilation event. Time-of-flight (TOF) PET detectors use precision in the detected timing to further localize the annihilation event along the LORs.
Detectors typically include scintillation crystals of a three-dimensional rectangular shape, which receive the gamma photons through a surface of the crystal facing the center of the detector ring. The gamma photon interacts with the molecules within the crystal, which converts the gamma photon or scintillates to generate luminescence. The generated luminescence or light is sensed by an optical sensor located on an opposite crystal face from the center facing surface. The crystals are sized of a sufficient dimension between the center facing and the sensor facing surface, such that the scintillation will occur somewhere in between the surfaces defined as a depth of interaction (DOI). For example, the crystals are typically long, rectangular bars with the smaller end center facing surface receiving the gamma photon, and with the opposite smaller end surface coupled to one or more optical sensors, which convert the sensed luminescence to an energy value and a time value measuring the received gamma photon. The scintillation or DOI occurs according to a distribution along the depth or length between the center facing surface and the optical sensor coupled facing surface, which varies by crystal. Systems typically use a fixed center point between the center facing surface and the coupled facing surface of the scintillation crystals for each end point of the LORs, which introduces error into the LOR, such as parallax error.